The present invention pertains to a device and method for detecting targets in aerosols using active optical sensors. Used in this context, aerosols means any suspension of solid or liquid particles in air or a gas, such as: mist, fog, dust, clouds. In the prior art, optical sensors are associated with explosive projectiles and are capable of activating circuitry within the projectile which arms or detonates the projectile when it is in proximity to a target. The device generally comprises a source of radiation which directs a narrow beam of radiation towards a target, and a receiver or detector which responds to radiation reflected by the target to the device. The detector provides an output signal in response to the radiation incident thereto. When this output signal reaches a threshold level, the projectile is armed or detonated.
One of the major difficulties with the optical sensor is its inability to discriminate between solid targets and aerosols. When the radiation beam is transmitted, if there are aerosols, between or covering the target, these aerosols will reflect some of the beam. These spurious reflections are picked-up by the sensors causing a premature detonation. In prior art this problem is rectified by trying to minimize the reflections caused by the aerosols. This is accomplished by desensitizing the sensors, a common method of which is to place an opaque mask or field stop in front of the detector. An apperture in the field stop is selectively positioned or sized so as to allow only a particular portion of the reflected radiation to reach the detector. This gives the detector a selected range while preventing short range spurious reflections from aerosols. This has been modified by using a plurality of appertures for different distances. These have some effect in reducing the spurious reflections from aerosols between the target and the projectile but cannot sort out the signal of a target when it is inside a cloud of aerosols.